当读写大于两个字的时候，报错如下：
2015-5-11 09:06:38 Default User modbus Serial Unable to read block address [400001 to 400003] on device 'Channel1.Device1'. Unexpected characters in response.
2015-5-11 09:06:50 Default User modbus Serial Device 'Channel1.Device1' is not responding.
2015-5-11 09:06:56 Default User modbus Serial Communications error on COM1 [*F*****]

我没有PC/PPI电缆。用有源RS485转RS232和无源RS485转RS232转换器都试过